<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <script>
    function sleep(sec){
      return new Promise((resolve, reject)=>{
        setTimeout(function (){
          resolve();
        }, sec*1000);
      })
    }

    async function show(){
      alert('a');

      await sleep(1);

      alert('b');

      await sleep(2);

      alert('c');
    }

    show();
    </script>
  </head>
  <body>

  </body>
</html>
